1. Field of the Invention
The present invention relates to a technique of generating halftone dots for color printing according to an image signal, and more particularly to prevention of moire in a color print.
2. Description of the Related Art
FIG. 1 shows a relationship between an output resolution Dp [dpi] (dots/inch) of halftone dots and their screen ruling Dh [lpi] (lines/inch). The output resolution Dp represents a number of pixels per inch in an output image generated by an output device such as an image recorder or a printer. As shown in FIG. 1, the output resolution Dp is equal to the reciprocal of a width of one pixel PX, or the reciprocal of a pixel pitch. The screen ruling Dh represents a number of halftone dots per inch in a halftone dot image. As shown in FIG. 1, the screen ruling Dh is equal to the reciprocal of the width of a one-dot area HA, or the reciprocal of a halftone dot pitch. The `one-dot area HA` here denotes an area in which one halftone dot is formed.
As shown in the bottom of FIG. 1, a number of pixels N included in one one-dot area HA is expressed by Equation (1) given below: EQU N=Dp.sup.2 /Dh.sup.2 (1)
A number of tones M reproducible by one halftone dot is not greater than the number of pixels N included in one one-dot area HA, as expressed by the following Inequality (2): EQU M.ltoreq.N (2)
Equation (1) and Inequality (2) show that a relationship defined by the following Inequality (3) is held between the number of tones M reproducible by one halftone dot, the output resolution Dp, and the screen ruling Dh: EQU Dh*.sqroot.M.ltoreq.Dp (3)
Inequality (3) gives the required output resolution Dp to achieve the desired screen ruling Dh and the desired number of tones M. By way of example, in order to achieve the screen ruling Dh of 175 lpi and M=256 tones, Inequality (3) indicates that the output resolution Dp is required to be not less than 2800 dpi.
The output resolution Dp is usually set equal to 4000 dpi in the conventional high-quality printing process. This output resolution Dp can sufficiently express 256 tones in each one-dot area HA. It is here assumed that the level of the image signal is in the range of 0 to 255. At least one pixel is recorded or lit up in each one-dot area HA when the level of the image signal is equal to 1. When the level of the image signal is equal to 254, at least one pixel remains unrecorded or in a non-lit-up state in each one-dot area HA
Recently, demand for high-speed image output has been increasing, and it is desired to lower the output resolution Dp while maintaining the number of tones M. AS clearly shown by Inequality (3) given above, however, the number of tones M reproducible by one halftone dot decreases with a decrease in the output resolution Dp.